This invention relates to the apparatus and method for surface treating the interior surfaces of hollow plastic objects to increase the surface tension level of the interior surfaces thereby improving the adhesive properties of these surfaces. More particularly, the apparatus and method by which interior surfaces are treated by creating a vacuum inside a hollow object, introducing a suitable gas therein and using an electrical discharge to ionize the gas to improve the surface tension and thereby the adhesive ability of the interior surfaces.
Adhesion of any substance (such as coatings, foams, glues, etc.) to the interior walls of hollow plastic objects (such as rotational molded parts, blown molded parts, etc.) has been a problem due to low surface energy level (surface tension) inherent in most plastics. The ability to adhere a substance to the interior walls of hollow plastic objects is highly desirable in numerous instances, such as foam insulation for strength of the object and insulation or coatings to make the interior surfaces less permeable to various products. For instance, it has been particularly difficult to adhere polyurethane foams of the standard types (using cloroflorocarbons (CFC) and hydrocloro-florocarbons (HCFC)) to the interior surfaces of hollow plastic objects. The introduction of poly foams with reduced CFC's or without CFC's (such as water borne foams) has proven particularly difficult to adhere to the interior surfaces without proper surface treatment. Standard methods of surface treatment (such as flame treating or corona discharge) have required that the plastic object be of two piece construction and that the interior surfaces of the objects be treated prior to final assembly. Other methods, such as gas plasma also have proven marginally effective in treating completely the interior surfaces. In addition, the aforementioned methods have proven to be costly and it has been difficult to treat irregularly shaped surfaces.